1. Field of the Invention
The invention generally relates to imaging devices and particularly to integrated circuit imaging devices requiring compensation for process and other variations.
2. Description of Related Art
Integrated circuit imaging devices include an array of light detecting elements interconnected to generate analog signals representative of an image illuminating the device. One common example of an integrated circuit imaging device is a charge coupled device (CCD) which is relatively expensive and consumes a relatively large amount of power. An alternative integrated circuit imaging device employs complementary metal oxide semiconductor (CMOS) image sensing elements. Within such an integrated circuit, a CMOS photo diode or photo transistor is employed as a light detecting element. In one example, conductivity of the element varies in accordance with the intensity of light illuminating the element. In other example, charge is collected in accordance with the intensity of light illuminating the element. By conducting current through the element or storing charge, an analog signal is thus generated having a magnitude approximately proportional to the intensity of light illuminating the element. CMOS integrated circuit imaging devices are considerably cheaper than CCD-type devices and may consume less power.
Both the CCD and CMOS integrated circuit imaging devices may require compensation for differences caused by variations within the integrated circuits, such as process, temperature, manufacturing, or voltage variations. For example, two different CCD elements within a single integrated circuit may generate analog signals of differing magnitudes for the same intensity of light illumination. Likewise, two CMOS photo diodes or photo transistors may generate analog signals of differing magnitudes when illuminated by light of equal intensity. As such, when exposed to a flat light image having equal intensities throughout, typical CMOS or CCD arrays may output analog signals having significant magnitude variations. Accordingly, if the analog signals are used to reproduce the original image, the reproduced image will not match the original flat image but will include a significant amount of variability and noise.
To eliminate this problem, imaging devices incorporating CCDs or CMOS photo diodes or photo transistors often include a compensation system for detecting and compensating for differences among the individual imaging elements. In one arrangement, a separate integrated circuit is provided with a random access memory (RAM). The integrated circuit containing the CCD or CMOS imaging elements is exposed to a flat image. Analog signals generated therefrom are converted to digital signals and binary compensation values representative of those signals are calculated and stored within the RAM. Thereafter, during use, analog signals received from the CCD or CMOS imaging array are converted to digital signals then adjusted in accordance with the binary values stored within the corresponding RAM array to compensate for any inherent variations between the sensing elements to thereby eliminate the aforementioned variability and noise. As such, if once again exposed to flat light, analog images output from the overall system are adjusted to match the flat input image.
Typically, such systems include, in addition to the integrated circuit imaging array and the separate RAM array, a separate controller circuit and a separate image processing logic circuit. The controller circuit controls generation of the compensation values for storage within the RAM array and the application of those values for adjusting subsequent images. The controller may also provide timing signals for the proper clocking of the imaging array. The image processing logic may include, for example, logic for filtering the image or otherwise manipulating the image to perform pattern recognition and the like.
Thus, typical integrated circuit imaging devices often include separate integrated circuits for the imaging array, the RAM, the compensation controller and the image processing logic. The provision of separate integrated circuits for the various components, and corresponding separate packaging elements, results in a fairly high cost for the overall system. Moreover, the need to transmit signals from one integrated circuit to the other can result in a relatively slow processing times, particularly for imaging arrays having a large number of elements such as a 1024-by-1024 array and higher.
It would be desirable to provide an improved integrated circuit imaging system which overcomes these disadvantages.